Fix dual table query when system schema is selected database

go/test/endtoend/vtgate/system_schema_test.go

@@ -139,19 +139,20 @@ func TestConnectWithSystemSchema(t *testing.T) {
 		connParams.DbName = dbname
 		conn, err := mysql.Connect(ctx, &connParams)
 		require.NoError(t, err)
+		exec(t, conn, `select @@max_allowed_packet from dual`)
 		conn.Close()
 	}
 }
 
 func TestUseSystemSchema(t *testing.T) {
 	defer cluster.PanicHandler(t)
 	ctx := context.Background()
+	conn, err := mysql.Connect(ctx, &vtParams)
+	require.NoError(t, err)
+	defer conn.Close()
 	for _, dbname := range []string{"information_schema", "mysql", "performance_schema", "sys"} {
-		conn, err := mysql.Connect(ctx, &vtParams)
-		require.NoError(t, err)
-
 		exec(t, conn, fmt.Sprintf("use %s", dbname))
-		conn.Close()
+		exec(t, conn, `select @@max_allowed_packet from dual`)
 	}
 }
 

go/vt/vtgate/planbuilder/plan_test.go

@@ -252,6 +252,20 @@ func TestWithDefaultKeyspaceFromFile(t *testing.T) {
 	testFile(t, "call_cases.txt", testOutputTempDir, vschema, false)
 }
 
+func TestWithSystemSchemaAsDefaultKeyspace(t *testing.T) {
+	// We are testing this separately so we can set a default keyspace
+	testOutputTempDir, err := ioutil.TempDir("", "plan_test")
+	require.NoError(t, err)
+	defer os.RemoveAll(testOutputTempDir)
+	vschema := &vschemaWrapper{
+		v:          loadSchema(t, "schema_test.json"),
+		keyspace:   &vindexes.Keyspace{Name: "mysql"},
+		tabletType: topodatapb.TabletType_MASTER,
+	}
+
+	testFile(t, "sysschema_default.txt", testOutputTempDir, vschema, false)
+}
+
 func TestOtherPlanningFromFile(t *testing.T) {
 	// We are testing this separately so we can set a default keyspace
 	testOutputTempDir, err := ioutil.TempDir("", "plan_test")
@@ -367,8 +381,8 @@ func (vw *vschemaWrapper) FindTableOrVindex(tab sqlparser.TableName) (*vindexes.
 	if err != nil {
 		return nil, nil, destKeyspace, destTabletType, destTarget, err
 	}
-	if destKeyspace == "" && vw.keyspace != nil {
-		destKeyspace = vw.keyspace.Name
+	if destKeyspace == "" {
+		destKeyspace = vw.getActualKeyspace()
 	}
 	table, vindex, err := vw.v.FindTableOrVindex(destKeyspace, tab.Name.String(), topodatapb.TabletType_MASTER)
 	if err != nil {
@@ -377,6 +391,20 @@ func (vw *vschemaWrapper) FindTableOrVindex(tab sqlparser.TableName) (*vindexes.
 	return table, vindex, destKeyspace, destTabletType, destTarget, nil
 }
 
+func (vw *vschemaWrapper) getActualKeyspace() string {
+	if vw.keyspace == nil {
+		return ""
+	}
+	if !sqlparser.SystemSchema(vw.keyspace.Name) {
+		return vw.keyspace.Name
+	}
+	ks, err := vw.AnyKeyspace()
+	if err != nil {
+		return ""
+	}
+	return ks.Name
+}
+
 func (vw *vschemaWrapper) DefaultKeyspace() (*vindexes.Keyspace, error) {
 	return vw.v.Keyspaces["main"].Keyspace, nil
 }

go/vt/vtgate/planbuilder/testdata/sysschema_default.txt

@@ -0,0 +1,17 @@
+# max_allowed_packet
+"select @@max_allowed_packet from dual"
+{
+  "QueryType": "SELECT",
+  "Original": "select @@max_allowed_packet from dual",
+  "Instructions": {
+    "OperatorType": "Route",
+    "Variant": "SelectReference",
+    "Keyspace": {
+      "Name": "main",
+      "Sharded": false
+    },
+    "FieldQuery": "select @@max_allowed_packet from dual where 1 != 1",
+    "Query": "select @@max_allowed_packet from dual",
+    "Table": "dual"
+  }
+}

go/vt/vtgate/vcursor_impl.go

@@ -282,7 +282,7 @@ func (vc *vcursorImpl) FindTableOrVindex(name sqlparser.TableName) (*vindexes.Ta
 		return nil, nil, "", destTabletType, nil, err
 	}
 	if destKeyspace == "" {
-		destKeyspace = vc.keyspace
+		destKeyspace = vc.getActualKeyspace()
 	}
 	table, vindex, err := vc.vschema.FindTableOrVindex(destKeyspace, name.Name.String(), vc.tabletType)
 	if err != nil {
@@ -291,6 +291,17 @@ func (vc *vcursorImpl) FindTableOrVindex(name sqlparser.TableName) (*vindexes.Ta
 	return table, vindex, destKeyspace, destTabletType, dest, nil
 }
 
+func (vc *vcursorImpl) getActualKeyspace() string {
+	if !sqlparser.SystemSchema(vc.keyspace) {
+		return vc.keyspace
+	}
+	ks, err := vc.AnyKeyspace()
+	if err != nil {
+		return ""
+	}
+	return ks.Name
+}
+
 // DefaultKeyspace returns the default keyspace of the current request
 // if there is one. If the keyspace specified in the target cannot be
 // identified, it returns an error.